Piezoelectric resonators have been used as micro-gravimetric immunoassay devices (See, for example, Joy E. Roederer and Glenn J. Bastiaans, "Microgravimetric Immunoassay with Piezoelectric Crystals", Anal. Chem. 1983, 2333-2336). Changes in the amount of mass attached to the surface cause shifts in the resonant frequency. Selective mass detection is achieved by coating the surface of the piezoelectric crystal with a substance that preferentially binds to the substance to be detected. For example, when specific antibodies are bound to the crystal surface, they selectively bind to their corresponding antigen. The concentration of the antigen in a fluid can be measured by immersing the sensor and inferring the change in mass from the change in resonant frequency.
The mass sensitivity (i.e., the fractional frequency change divided by the causative surface mass density change) increases as the mass of a bulk wave resonator is decreased or, correspondingly, as the resonator thickness is decreased. A practical lower limit of about 100 microns, corresponding to a resonance frequency of about 20 MHz, is imposed on resonator thickness by manufacturing difficulties. Consequently the sensitivity of a bulk wave resonator sensor is limited.
Acoustic waveguide devices have also been used as mass sensors. Mass attached to the surface of the waveguide will produce a phase shift in the output signal from the waveguide. The concentration of the antigen can be determined from this phase shift. These devices can utilize different wave motions, including Rayleigh waves (SAWs), Lamb waves, surface transverse waves (STWs) and surface-skimming bulk waves (SSBWs). In Rayleigh and Lamb wave devices, the predominant component of particle motion due to acoustic displacement is normal to the surface of the waveguide and is thereby also normal to the direction of propagation. In STW devices, the predominant particle velocity component due to acoustic displacement is parallel to the surface and normal to the propagation direction. SSBWs have the same dominant particle velocity direction as STWs, but the acoustic waves are not trapped at the surface, as is the case in STWs, and therefor diffract into the substrate.
The particle velocity direction due to acoustic displacement has a strong influence on the behavior of the device when immersed in a fluid. The acoustic wave propagation velocity varies as the square root of the ratio of the aggregate stiffness to the aggregate mass of the waveguide. The addition of mass to the surface of a waveguide decreases the propagation velocity, which causes a corresponding change in the net phase shift across the device. The phase shift can be measured directly, or inferred from measurements of the frequency of an oscillator containing the surface acoustic wave device as a feedback element.
In a surface wave device, since the wave is trapped within a region near the surface of the device, the response of this wave to additional mass on the surface is dependent only on the amount of mass within the region within which the wave is trapped. Therefore, the fractional mass sensitivity of a surface wave device is a function of the penetration depth of the wave into the waveguide instead of the substrate thickness.
The penetration depth ranges from a fraction of an acoustic wavelength to a few acoustic wavelengths, depending on the characteristics of the device and the type of wave motion. Because acoustic waveguide devices can be easily fabricated at frequencies of up to several gigahertz using standard photolithographic techniques, the penetration depths can be reduced to the order of several microns, thereby making these devices very sensitive.
Although the fractional frequency (or phase) change induced in an acoustic waveguide device is strictly a function of the penetration depth of the wave, the phase change is proportional to the length of the device. The optimum length depends on the degree of wave attenuation. This sensitivity dependence on length provides a further sensitivity advantage over a bulk wave resonator.
SAW devices make poor chemical sensors in applications which require the immersion of the sensor in a liquid because the dominant acoustic displacement component couples strongly to compressional waves in the fluid. The reason for this is as follows. Since the acoustic propagation velocities of Rayleigh waves in solids are almost universally higher than the velocities of compressional waves in liquids, there always exists a direction of radiated acoustic waves in the liquid that at the surface of the waveguide are in phase with the Rayleigh mode of the SAW device. Consequently energy will radiate away from the SAW device into the fluid, causing an unacceptable amount of insertion loss.
Lamb wave devices also make poor chemical sensors for a related reason (See, for example, R. M. White, P. J. Wicker, S. M. Wenzel, and E. T. Zellers, "Plate Mode Ultrasonic Oscillator Sensors", IEEE Trans., vol. UFFC-34, #2, pp. 163.). Lamb wave devices have the same dominant acoustic particle velocity component as SAW devices. However, by decreasing the thickness of the waveguide, the velocity of the mode can be caused to fall below that of the surrounding fluid. This prevents phase matching and hence prevents the radiation of energy into the liquid. Unfortunately, this decreased thickness also causes the velocity of the Lamb wave to be a strong function of the density of the fluid. This effect masks the mass sensitivity of the sensor. In addition, the difficulties of fabricating a thin waveguide limit the maximum frequency of a Lamb wave device to several Megahertz, thereby limiting the sensitivity of such a device. STW devices do not suffer from these limitations, as the summary of the invention will show.